The art of bulk micromachining in silicon has changed since the invention of practical means to etch vertical sidewalls in silicon using dry etching technology, as has largely resulted from the invention of the BOSCH process. Deep dry etching technology has allowed new architectures in single crystal silicon to be created, especially in released structures such as those based on SOI silicon like accelerometers and electrostatic actuators. While such deep dry etch processes have allowed the realization of many new architectures, there are still advantages in traditional crystallographic (anisotropic) etching in silicon. Wet anisotropic etching is often based on opening a hard mask deposited on silicon, with features oriented on the surface to create v-grooves, u-grooves, precision inverted pyramidal pits, and other shapes, which are well known in the art. The exact shapes depending on the crystal orientation, mask opening shape, and particular wet etch used. Advantages of wet etching, with proper alignment and etch conditions, include the ability to batch process many wafers at one time, the ability to achieve very smooth surfaces, and the ability to achieve non perpendicular surfaces such as v-grooves, and the ability to produce highly accurate mechanical dimensions. Alternatively, the use of deep plasma etching has the advantage of being independent of crystallographic axis limitations and allows vertical surfaces to be created with high aspect ratio. What is lacking in the art is a method to combine these two etching formats so that the benefits of both techniques can be brought together allowing new freedom in possible resulting shapes and structures. Combining these to etching formats would be particularly useful for the art of silicon optical bench, where elements such as micro-optics, semiconductor lasers, photodetectors, optical fibers, and other elements can be hybridly integrated on the silicon wafer surface using mechanical features etched into the silicon, along with integrated patterned metals, solders, resistors, and MEMS that can be fabricated directly into or onto the silicon wafer. Thus microsystems can be achieved with assembly economy difficult to otherwise achieve. It should be clear that other such wafer level micro-systems will clearly benefit from this improvement in micromachining technology such as sensors, actuators, micro-fluidics, RF microdevices, and so on. For example, can apply the methods of the present invention to the fabrication of known bulk micromachined products such as accelerometers to create architectures leveraging dry and wet etching. By way of example and not limitation, the instant disclosure describes mechanical structures which can be realized and that are useful in hybrid micro-optical electrical systems, also known as silicon optical bench, SiOB, silicon wafer board, or simply silicon bench.
The ability to precisely locate optical elements relative to one another is of critical importance in the fabrication of micro-optical devices, since the alignment tolerances between elements are often specified in submicron dimensions. Typically, such elements may include an optical signal source, such as a laser, a detector, and an integrated or discrete waveguide, such as a fiber-optic, integrated optics, or GRIN rod lens. Additionally, such elements may include a fiber amplifier, optical filter, modulator, grating, ball lens, or other components for conveying or modifying or splitting an optical beam. Micro-optical devices containing such components are crucial in existing applications such as optical communication and consumer opto-electronics, as well as applications currently being developed, such as optical computing.
Maintaining precise alignment among the optical elements may be conveniently provided by an optical microbench, such as a silicon optical bench. An optical microbench comprises three-dimensional structures having precisely defined surfaces onto which optical elements may be precisely positioned. One material well-suited for use as an optical microbench is single crystal silicon, because single crystal silicon may be etched anisotropically to yield three-dimensional structures having planar sidewalls formed by the precisely defined crystallographic planes of the silicon. For example, the {111} silicon plane is known to etch more slowly than the {100} or {110} planes with proper choice of etchant. Thus, structures may be formed comprising walls that are primarily {111} planes by anisotropic etching.
Since the many optical elements sit within the three-dimensional structures at a position at least partially below a top surface of the silicon substrate, a portion of the optical path often lies below the top surface of the substrate, within the volume of the substrate. Accordingly, passageways must be provided in the optical microbench between three-dimensional structures so that light may travel between the elements disposed in the associated three-dimensional structures. Hence, an optical microbench should contain three-dimensional structures that communicate with one another through structures such as a passageway.
While discrete, non-communicating, three-dimensional structures may be conveniently formed by an anisotropic etching, etched structures which communicate with one another at particular geometries, such as a convex corner, pose significant problems for applications in which it is desirable to maintain the precise geometry defined by the crystallographic planes. For example, where two {111} planes intersect at a convex (or exposed) corner, the convex corner does not take the form of a straight line intersection between two planes, but is rather rapidly attacked by the etchant to create a rounded or complex intersection between the two {111} planes. As etching continues to reach desired depth of the structure containing the {111} planes, the rounding or attack of the corners can grow to such an extent that a substantial portion of the intersection between the two {111} planes is obliterated. Since the {111} planes are provided in the three-dimensional structures to form a planar surfaces against which optical elements may be precisely positioned, absence of a substantial portion of the {111} planes at the intersection can introduce a great deal of variability of the positioning of the elements at the intersection. Thus, the benefits provided by the crystallographic planes can be unacceptably diminished.
Traditionally, to avoid etching intersecting features, dicing saw cuts may be used. However, dicing saw cuts can be undesirable, because such cuts typically must extend across the entire substrate, or consume an undesirably large portion of it, and may not conveniently be located at discrete locations within the substrate. Moreover, dicing saw cuts create debris which may be deposited across the substrate surface and lodge within the three-dimensional structures, which may interfere with the precise positioning of optical elements within such a structure.
Therefore, there remains a need in the art for optical microbench technology which permits three-dimensional structures having crystallographic planar surfaces to intersect with other surfaces, without degrading the crystallographic orientation of the intersected planar surfaces. Further, there remains a need in the art to combine crystallographic surfaces and vertical dry etched surfaces together in the same structure.